5-Chlorodeoxycytidine (CldC) coadministered with tetrahydrouridine (H4U) will be utilized to sensitize transplanted murine tumors to X-ray. We have achieved a 3.4- to 3.8-dose increase effect with X-ray utilizing PALA + F-pyrimidine treatment prior to CldC + H4U in cell culture. Enzyme kinetic, DNA incorporation and cell culture studies indicate that C1dC is converted to CldCMP by deoxycytidine kinade and deaminated by dCMP deaminase which is markedly elevated (10- to 80-fold) in mouse and human tumors, to form CldUMP. The 4-amino group protects the nucleoside analog from catabolism by nucleoside phosphorylases. Thus, we are using a storage form of a nucleoside analog that we propose is converted preferentially at the tumor site to a cytotoxic as well as radiosensitizing agent. BrdC can not be used this way because of its high Km value with respect to dC kinase. The following are studies which will be undertaken with tumor and normal tissue of the mouse to determine optimum protocols for the radiation of transplanted tumors. A. Incorporation of 3H-C1dU derived from 3H-C1dC, B. Pharmacokinetic and tissue disposition of 3H-C1dC metabolites, C. Cytokinetic, D. Enzymatic and E. Pool sizes of normal metabolites. Extensive incorporation studies will be undertaken prior to irradiation of Sarcoma-180 and Lewis lung carcinoma. Four derivatives of one of these tumors that are deficient in cytidine- or dCMP-deaminase or deoxycytidine- or thymidine-kinase will be studied to test the validity of our hypothesis. We will investigate deleterious effects of C1dC + H4U administration (e.g. sister chromatid exchange) and, if encountered, we will rescue by thymidine or deoxycytidine (dC) administration immediately following radiation. In other approaches to achieve sensitization we will utilize BrdC or C1dC + low concentrations of H4U in tumors with high levels of cytidine deaminase. C1dC + 2' dH4U, and inhibitor of dC and dCMP deaminase, will be utilized to obtain incorporation of C1dC as such into DNA. Our major strategy goes beyond the exploitation of differences in cell kinetics between normal and neoplastic tissues; by taking advantage of quantitative differences in the levels of enzymes, we seek to obtain preferential conversion of C1dC to C1dUTP at the tumor site. Because the mechanism of pyrimidine nucleoside radiosensitization is different from hyperthermy and the action of hypoxic cell radiosensitizers, our approach may have potential in combination with those modalities.